1. Field of the Invention
The present invention relates to a system and method for improving Time Of Arrival (TOA) distance measurements between nodes of a wireless ad-hoc network. Specifically, the present invention relates to a system and method for performing distance estimation using square-root raised cosine pulse shaping and chip matched filters on direct sequence spreading waveforms, the multiplication of which produces a raised cosine filtered pulse response. The response is used to identify a time when a function is at a maximum, corresponding to the actual signal reception time.
2. Description of the Related Art
Wireless communication networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks are commonly referred to as “cellular networks”, because the network infrastructure is arranged to divide the service area into a plurality of regions called “cells”. A terrestrial cellular network includes a plurality of interconnected base stations, or base nodes, that are distributed geographically at designated locations throughout the service area. Each base node includes one or more transceivers that are capable of transmitting and receiving electromagnetic signals, such as radio frequency (RF) communications signals, to and from mobile user nodes, such as wireless telephones, located within the coverage area. The communications signals include, for example, voice data that has been modulated according to a desired modulation technique and transmitted as data packets. As can be appreciated by one skilled in the art, network nodes transmit and receive data packet communications in a multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at the base node to communicate simultaneously with several mobile nodes in its coverage area.
In recent years, a type of mobile communications network known as an “ad-hoc” network has been developed for use by the military. In this type of network, each mobile node is capable of operating as a base station or router for the other mobile nodes, thus eliminating the need for a fixed infrastructure of base stations. Details of an ad-hoc network are set forth in U.S. Pat. No. 5,943,322 to Mayor, the entire content of which is incorporated herein by reference.
More sophisticated ad-hoc networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in a conventional ad-hoc network, further enable the mobile nodes to access a fixed network and thus communicate with other mobile nodes, such as those on the public switched telephone network (PSTN), and on other networks such as the Internet. Details of these advanced types of ad-hoc networks are described in U.S. patent application Ser. No. 09/897,790 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks”, filed on Jun. 29, 2001, in U.S. patent application Ser. No. 09/815,157 entitled “Time Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channel”, filed on Mar. 22, 2001, and in U.S. patent application Ser. No. 09/815,164 entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile Radio Access System”, filed on Mar. 22, 2001, the entire content of each being incorporated herein by reference.
The mobile nodes of such networks may assume any number of random positions within the network, making exact node location determinations difficult when needed. For computing node geographical coordinates in such ad-hoc wireless networks, algorithms in use at individual nodes in typical networks use the “Time Of Arrival” (TOA) measurement technique. As can be appreciated by one skilled in the art, a TOA measurement provides the distance between mobile nodes for computing a mobile node position. The measurements are based upon signal propagation times, specifically the time a signal needs for traveling between transceivers of a target node and a reference node.
Time Of Arrival (TOA) measurements provide an estimate of the distance between two transceivers, or nodes. In order to perform high precision computations for mobile node location services, it is necessary to measure the distance between two transceivers with a high degree of precision. One technique used to estimate the distance between two transceivers is based on the computation of the auto-correlation function of a spreading sequence, and only yields a precision of ±0.5 Tc (where Tc is the chip rate) or ±4.68 meters (for a chip rate of 32 Mcps).
Profile estimations may also be achieved using a quadratic function. For instance, the multipath profile of a radio frequency (RF) signal can be approximated using a quadratic function and multiple signal measurements. As described in U.S. Pat. No. 6,269,075 to Tran, the entire content of which is incorporated herein by reference, the peak of the quadratic function, corresponding to the multipath with the peak signal strength, can be found through a simple derivation. However, as further detailed in the Tran Patent, non-quadratic shapes can require a correction curve derived from actual profile information, from which an estimated timing offset can be calculated. However, Tran fails to disclose that the multipath profile of the RF signal is related to the pulse-shaping filters used in the transceiver, and that this property can be used to determine the actual correction to apply.
Accordingly, a need exists for a system and method to provide an accurate estimate of a signal time of arrival and timing offset correction for non-quadratic signal profiles using pulse-shaping filter outputs in order to perform high precision computations for mobile unit location services.